A two-phase motor (such as a permanent magnet synchronous motor and induction motor) can be used in many applications to provide power assistance to mechanical systems. It is known to control the phase winding voltages in a two-phase motor using pulse width modulated signals. The pulse width modulated signals are applied to an inverter or a series of switching devices that connect the phase windings of the motor to a positive and negative terminal of a direct current (DC) power supply, such as a battery. This inverter consists of four switches in an upper bank and four switches in a lower bank.
To adequately control the motor and minimize torque ripple, it is necessary to measure the current flowing through each phase winding. The current measured in each phase is provided to a controller that generates the pulse width modulated signals. To measure currents, it is known to use a resistor in series with each phase winding. The voltage drop across each resistor is measured to determine the current flowing for each phase. This type of system has the disadvantage of requiring two current sensors, one for each of the two phase windings, and two measurement circuits. These existing configurations use either Hall Effect sensors or shunts, which are expensive. Moreover, Hall Effect sensors have differing response characteristics due to sensor accuracy, amplification offset, gain and linearity, and temperature and aging drift, which will cause a mismatch between the two current channels which generates torque ripple. This is also true for shunts or any other current sensor that requires an amplification stage. Further, there is an issue with common-mode noise rejection in two-phase motors that is not addressed.
It is also known to use a single resistor to perform the function of measuring the current through each phase of a Y-connected three-phase motor. The single resistor is located external to the motor windings on the DC link between the DC power supply and the inverter or series of switching devices. Depending on the states of the switches and period of operation, the currents through the phases may be measured or calculated by the single resistor. Specifically, in a Y-connected three-phase motor, the sum of all currents flowing through all phases of the motor are zero, so by knowing the current flowing through one or two phases a determination of the current flowing through the remaining phase(s) may be made (with the exception of Y-connected motors with a grounded center point or a motor with a fault). However, this solution is not applicable to two-phase motors without a common winding connection.
It is also known to close either the upper bank of switching devices or the lower bank of switching devices, thereby recirculating the current in the windings of the motor together when damping or braking is required. Braking is used to damp unwanted mechanical system oscillations and is also used to reduce the speed of the mechanical system. For example, mechanical resonances or disturbances may feed back into the motor from the mechanical system. Not only are unstable mechanical resonances undesired by themselves, these resonances can generating an electromotive force (EMF) that can result in excess current in the motor. In damping mode, the closed switches allow current to circulate within the motor windings to act as a brake to oppose any motion induced by the mechanical resonances, thereby helping to restore stability. However, the internal circulation of currents within the motor windings in this mode cannot be measured by a single external resistor since the resistor is not in a current carrying loop. Therefore, the currents within the motor windings cannot be measured, and therefore cannot be modified or controlled.
Recent advances in digital signal processors (DSPs) have permitted the use of more advanced pulse width modulation schemes such as space vector pulse width modulation (hereinafter “SVPWM”). One significant advantage of using SVPWM in a 3-phase system is that it can provide more output voltage compared to conventionally known sinusoidal pulse width modulation schemes. The drawback, however, is that SVPWM requires more complex schemes to generate PWM signals applied to the inverter. The main challenge into measuring full phase currents via a single DC link current sensor is that there are situations where the full phase currents cannot be sampled during SVPWM operation. One situation is when the amplitude of the voltage space vector is very small. Another situation is when the voltage space vector falls on the other active vector.
Therefore, a need exists for an improved technique to monitor current in a two-phase device, using a single current sensor. It would also be of benefit if common-mode noise rejection can be provided. Further, it would be desirable to provide this technique with no additional hardware requirements, thereby improving reliability and saving cost.
While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the broad scope of the invention as defined by the appended claims.